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RedHat 9 (Linux i386) - man page for netlink (redhat section 7)

NETLINK(7)			    Linux Programmer's Manual			       NETLINK(7)

       netlink, PF_NETLINK - Communication between kernel and user.

       #include <asm/types.h>
       #include <sys/socket.h>
       #include <linux/netlink.h>

       netlink_socket = socket(PF_NETLINK, socket_type, netlink_family);

       Netlink	is  used to transfer information between kernel modules and user space processes.
       It consists of a standard sockets based interface for user processes and an internal  ker-
       nel  API  for  kernel modules. The internal kernel interface is not documented in this man
       page. Also there is an obsolete netlink interface  via  netlink	character  devices,  this
       interface is not documented here and is only provided for backwards compatibility.

       Netlink is a datagram oriented service.	Both SOCK_RAW and SOCK_DGRAM are valid values for
       socket_type; however the netlink protocol does not distinguish between  datagram  and  raw

       netlink_family  selects	the kernel module or netlink group to communicate with.  The cur-
       rently assigned netlink families are:

	      Receives routing updates and may be used to modify  the  IPv4  routing  table  (see

	      Receives packets sent by the IPv4 firewall code.

	      For managing the arp table in user space.

	      Receives and sends IPv6 routing table updates.

	      to  receive  packets  that  failed  the  IPv6 firewall checks (currently not imple-

	      are the instances of the ethertap device.  Ethertap  is  a  pseudo  network  tunnel
	      device that allows an ethernet driver to be simulated from user space.

	      Reserved for ENskip.

	      is reserved for future user space protocols.

       Netlink	messages consist of a byte stream with one or multiple nlmsghdr headers and asso-
       ciated payload.	For multipart messages the first  and  all  following  headers	have  the
       NLM_F_MULTI  flag set, except for the last header which has the type NLMSG_DONE.  The byte
       stream should only be accessed with the standard NLMSG_* macros, see netlink(3).

       Netlink is not a reliable protocol.  It tries its best to deliver a message to its  desti-
       nation(s),  but	may  drop messages when an out of memory condition or other error occurs.
       For reliable transfer the sender can request an acknowledgement from the receiver by  set-
       ting  the NLM_F_ACK flag.  An acknowledgment is an NLMSG_ERROR packet with the error field
       set to 0.  The application must generate acks for received messages  itself.   The  kernel
       tries  to send an NLMSG_ERROR message for every failed packet.  A user process should fol-
       low this convention too.

       Each netlink family has a set of 32 multicast groups.   When  bind(2)  is  called  on  the
       socket,	the  nl_groups	field in the sockaddr_nl should be set to a bitmask of the groups
       which it wishes to listen to.  The default value for this field is zero which  means  that
       no  multicasts  will be received.  A socket may multicast messages to any of the multicast
       groups by setting nl_groups to a bitmask of the groups it wishes to send to when it  calls
       sendmsg(2)  or  does  a	connect(2).   Only  users  with  an  effective	uid  of  0 or the
       CAP_NET_ADMIN capability may send or listen to a netlink multicast group.  Any replies  to
       a  message  received  for a multicast group should be sent back to the sending pid and the
       multicast group.

	      struct nlmsghdr
		  __u32    nlmsg_len;  /* Length of message including header */
		  __u16    nlmsg_type; /* Message content */
		  __u16    nlmsg_flags;/* Additional flags */
		  __u32    nlmsg_seq;  /* Sequence number */
		  __u32    nlmsg_pid;  /* PID of the process that opened the socket */

	      struct nlmsgerr
		  int	   error;      /* negative errno or 0 for acks. */
		  struct nlmsghdr msg; /* message header that caused the error */

       After each nlmsghdr the payload follows.  nlmsg_type can be one of  the	standard  message
       types:  NLMSG_NOOP  message is to be ignored, NLMSG_ERROR the message signals an error and
       the payload contains a nlmsgerr structure, NLMSG_DONE message terminates a multipart  mes-

       A  netlink  family usually specifies more message types, see the appropriate man pages for
       that, e.g.  rtnetlink(7) for NETLINK_ROUTE.

       Standard Flag bits in nlmsg_flags
       NLM_F_REQUEST   set on all request messages
       NLM_F_MULTI     the message is part of a multipart mes-
		       sage terminated by NLMSG_DONE
       NLM_F_ACK       reply with an acknowledgment on success
       NLM_F_ECHO      echo this request

       Additional flag bits for GET requests
       NLM_F_ROOT     Return the complete table instead of a single entry.
       NLM_F_MATCH    Not implemented yet.
       NLM_F_ATOMIC   Return an atomic snapshot of the table.
       NLM_F_DUMP     not documented yet.

       Additional flag bits for NEW requests
       NLM_F_REPLACE   Override existing object.
       NLM_F_EXCL      Don't replace if the object already exists.
       NLM_F_CREATE    Create object if it doesn't already exist.
       NLM_F_APPEND    Add to the end of the object list.

       Note that NLM_F_ATOMIC requires CAP_NET_ADMIN or super user rights.

       The  sockaddr_nl  structure  describes a netlink client in user space or in the kernel.	A
       sockaddr_nl can be either unicast (only send to	one  peer)  or	send  to  netlink  groups
       (nl_groups not equal 0).

	      struct sockaddr_nl
		  sa_family_t nl_family;    /* AF_NETLINK */
		  unsigned short nl_pad;    /* zero */
		  pid_t       nl_pid;	    /* process pid */
		  __u32       nl_groups;    /* multicast groups mask */

       nl_pid is the pid of the process owning the destination socket, or 0 if the destination is
       in the kernel.  nl_groups is a bitmask with every bit representing a netlink group number.

       This man page is not complete.

       It is often better to use netlink via libnetlink than via the low level kernel interface.

       The socket interface to netlink is a new feature of Linux 2.2

       Linux 2.0 supported a more primitive device based netlink interface (which is still avail-
       able as a compatibility option). This obsolete interface is not described here.

       cmsg(3), rtnetlink(7), netlink(3)

       ftp://ftp.inr.ac.ru/ip-routing/iproute2* for libnetlink

Linux Man Page				    1999-04-27				       NETLINK(7)

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